Communication devices such as wireless devices are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or Mobile Stations (MS). Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication network, sometimes also referred to as a cellular radio system, cellular system, or cellular network. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The wireless communications network covers a geographical area which may be divided into cell areas, each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g., evolved Node B (“eNB”), “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. Wide Area Base Stations, Medium Range Base Stations, Local Area Base Stations and Home Base Stations, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the wireless device. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Radio transmitters may emit Radio Frequency (RF) electromagnetic fields. A Radio Frequency (RF) ElectroMagnetic Field (RF EMF) may be understood as a physical field consisting of electric and magnetic field components produced by accelerating electrical charges. The electromagnetic energy propagates as waves where a changing electric field gives rise to a magnetic field and vice versa. The electromagnetic field strength may be understood as a magnitude of an electromagnetic field vector. With knowledge of the RF EMF, power density and Specific Absorption Rate may be determined. The field strength levels resulting from the RF EMF that may be emitted by radio transmitters, such as Radio Base Stations (RBS), may need to be controlled and maintained below certain limit values in different regions of space to comply with a regulation, that is, a law, policy or standard, for example. The regulation may be set due to various reasons.
One of the reasons is human safety. Human exposure to RF EMF may be subject to national and international regulations and standards, which in many countries may be based on recommendations from the World Health Organization (WHO) and the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [1]. One example of a regulation is found in Europe, where maximum RF exposure levels are specified in the Council Recommendation 1999/519/EC [2]. The ICNIRP guidelines specify basic restrictions and reference levels for occupational and general public exposure. In the frequency range of interest for existing mobile communication systems, the basic restrictions are expressed in terms of Specific Absorption Rate (SAR) and the reference levels as limits on electric and magnetic field strength or power density. SAR in watts/kilogram (W/kg) is a measure of the rate of RF energy absorption in tissue. For future 5G mobile communication systems, the use of higher frequency bands is of interest. Below 10 GHz, for ICNIRP, the basic restriction is specified in terms of SAR, as a quantity measured inside the body. As the frequency increases, the energy absorption in the human tissue becomes more superficial, and above 10 Gigahertz (GHz), the ICNIRP basic restrictions change from SAR to incident power density [1], that is, to a quantity measured without the body present.
RF EMF exposure assessments may be conducted with the purpose to make sure that the exposure levels from the considered Equipment Under Test (EUT) may not exceed the relevant limits in areas accessible to workers and members of the general public.
Before an RBS product may be placed on the market, an RF EMF product compliance assessment may be conducted, where a compliance boundary may be determined outside of which the exposure is below the exposure limits. These compliance boundaries may be determined using either calculations or measurements as if the products are transmitting in free space. For products using external antennas, the RF EMF compliance assessment may be normally conducted for a typical antenna. For products with internal antennas, the entire EUT may be considered for the exposure assessment. The size and shape of the compliance boundary depend on the output power and the EUT/antenna geometry, including material properties. The compliance boundary may be usually described with a simpler shape such as a rectangular box or a cylinder enclosing the EUT/antenna.
Operators putting an RBS into service may be required to conduct an RF EMF product installation compliance assessment. The main difference compared with the product compliance assessment is that contributions from possible ambient sources and/or the effect of scatterers may need to be considered. Methods for how to approximately consider effect of scatterers, and exposure levels above which contributions from ambient sources may need to be considered, have been standardized [3], [4].
For mobile terminals, there are currently methods aimed to direct a signal away from the body and keep RF exposure below established SAR limits. In one example this is based on sensing the reflected power in the antenna [6]. This is possible when the terminal is very close to the human body. In another example [5] directing a signal away from the body and keeping an RF exposure below established SAR limits is based on sensing the proximity of the user to the terminal or on sensing how the terminal is held, and subsequently directing the emission away from the user.
Another reason for why RF EMF levels may need to be controlled and maintained below certain limit values may be when the radio transmitters may be used in environments containing sensitive electrical or electronic equipment. This may e.g., include intensive care departments in hospitals or nuclear power plants with strict requirements on ElectroMagnetic Compatibility (EMC). Also for this case, compliance boundaries may be determined but instead using limits of relevance for EMC applications, e.g., the generic electric field strength immunity limit of 3 voltmeter (V/m).
Current wireless standards support multi-antenna transmission for beamforming, diversity, and spatial multiplexing through different precoding schemes. Beamforming may be understood as a signal processing technique for directional signal transmission or reception using antenna arrays. This may be achieved by multiplying the signals associated with each antenna element in the array with complex values (weights) to make the transmission/reception at particular angles experience constructive interference while others experience destructive interference. In closed-loop precoding, candidate beam shapes may be evaluated by a receiver based on reference signal transmissions. The candidate beam shapes may be available as a pre-agreed codebook of transmit antenna weights to be applied on different transmit antenna elements. The receiver may select a preferred codebook entry, and hence one or several beam shapes, and report this to the transmitter for use in data transmissions. There exists functionality where the transmitter may restrict the receiver to only consider a subset of the available beam shapes. Other precoding schemes such as e.g. reciprocity-based beamforming may allow much greater flexibility of the possible beam shapes.
Typically, the compliance boundary determined as indicated above may be understood as a fixed zone surrounding the EUT/antenna. To account for beamforming, where very high antenna gains may be achievable, a conservative scenario is normally used in existing methods, where the compliance boundary is made large enough to ensure that EMF levels are below the relevant limits on RF exposure or EMC for all possible antenna weights. This may lead to very large compliance distances which may hinder the use of beamforming or make it difficult to install the RBS product at the desired location.
There are other methods available to direct the beamforming away from any human presence or alternatively to reduce the output power in, typically used for mobile terminals. For a person skilled in the art, it may also be possible to use the same methods for radio base stations. The existing methods are based on sensors which will suffer from detection and reliability errors and may further require constant monitoring of the surroundings of the antenna. Many of the sensor types such as proximity sensors or reflected power sensors are also unsuitable to detect the presence of humans at larger distances such as relevant for an access point installation. The efficiency of such methods as in [5] and [6] are also difficult to assess in measurements or numerical calculations and hence they may not be sufficient to allow relaxation of the compliance distance in certain directions.
In summary, given the limits on radio frequency electromagnetic field strength set by regulations, the use of communication devices with existing methods is limited.